1. Field of the Invention
The present invention relates to a semiconductor light emitting device provided with a rough surface such that a film thickness of a light extract layer is not reduced.
2. Related Art
As to a light emitting diode (LED) which is a semiconductor light emitting device, it is possible to fabricate a high luminance LED emitting various color lights such as blue, green, orange, yellow, and red, since it is possible to grow a GaN based high quality crystal or an AlGaInP based high quality crystal by using MOVPE (Metal Organic Vapor Phase Epitaxy) method in recent years. In accordance with the provision of a high luminance LED, applications of the LED are widened, e.g. a back light of a liquid crystal display, a brake lump for a vehicle, so that demand for the LED increases year by year.
Since the growth of a high quality crystal by using the MOVPE method has been achieved, a light emitting efficiency inside the light emitting device is nearing to a theoretical limit value. However, the light extract efficiency from the light emitting device to the outside is still low, and enhancement of the light extract efficiency is expected.
For example, a high luminance red LED is made from AlGaInP based materials, and has a double hetero structure comprising a conductive GaAs substrate, an n-type AlGaInP layer comprising an AlGaInP based material with a composition which is lattice-matched with the conductive GaAs substrate, a p-type AlGaInP layer, and an active layer which is a part of a light emitting part comprising AlGaInP or GaInP, in which the active layer is sandwiched by the n-type AlGaInP layer and the p-type AlGaInP layer. The AlGaInP based material here is a general term of various kinds of materials mainly comprising AlGaInP, in that composition ratios or additives are different from each other. In the semiconductor light emitting device using the AlGaInP based material, materials such as GaInP, and GaP may be used together.
Since a bandgap of the GaAs substrate is narrower than that of the active layer in such a semiconductor light emitting device, most of the light emitted from the active layer is absorbed by the GaAs substrate, so that the light extract efficiency is deteriorated.
As means for solving this problem, there is a technique for improving the light extract efficiency by forming a layer with a multilayer reflective film structure comprising semiconductors having different refractive indices between the active layer and the GaAs substrate, to reflect the light emitted to the GaAs substrate, thereby reducing absorption of the light in the GaAs substrate. However, according to this technique, only the light having a limited incident angle with respect to the multilayer reflective film structure layer is reflected. In other words, only a part of the light emitted to the GaAs substrate is reflected, so that it is difficult to improve the light extract efficiency enough.
Thus, Japanese Patent Laid-Open No. 2002-217450 discloses another technique for realizing a high luminance by forming a semiconductor light emitting device in which a double hetero structure part comprising AlGaInP based material is grown on a GaAs substrate for growth, sticking the double hetero structure part on a supporting substrate comprising Si, GaAs or the like via a metal layer with a high reflectance, and removing the GaAs substrate used for the growth. According to this technique, since the metal is used as a reflective layer, the reflection with high reflectance can be realized without selecting an incident angle with respect to the reflective layer. For this reason, it is possible to provide a higher luminance than the aforementioned technique in which the multilayer reflective film structure is formed. In other words, it is possible to achieve the higher luminance by extracting the light generated in the active layer more effectively.
FIG. 11 is a schematic cross sectional view showing a structure of a conventional semiconductor light emitting device.
As shown in FIG. 11, a conventional semiconductor light emitting device 101 comprises, in an order from up to bottom, a first electrode 102 for partially covering a light extract layer, a first electrode side contact layer 103 provided only just beneath the first electrode 102 and covering a part of the light extract layer covered by the first electrode 102, the first electrode side contact layer 103 having a bandgap energy smaller than that of the active layer and being opaque with respect to the light emitted from the active layer, a light extract layer 104 constituting a main surface at a first cladding layer side and radiating the light advancing from the active layer to the first cladding layer side, a first cladding layer 105 that is one of two cladding layers sandwiching the active layer, an active layer 106 sandwiched by the first and second cladding layers and generating the light, a second cladding layer 107 that is another one of the two cladding layers, an interposed layer 108 interposed between the second cladding layer 107 and a reflective metal film side contact layer, a reflective metal film side contact layer 109, an oxide layer 110, a reflective metal film 111 provided between the second cladding layer 107 and a second electrode and reflecting the light advanced from the active layer 106 to a second electrode side, a metal adhesion layer 112, a supporting substrate 113 on which the double hetero structure part is stuck, and a second electrode 114 for covering an opposite surface with respect to the main surface.
The light extract layer 104 is also called as a window layer.
The oxide layer 110 comprises ohmic contact portions 115 dispersed appropriately in a plane contacting to the reflective metal film 111, in which parts other than the ohmic contact portions 115 are referred as non-ohmic contact portions 116.
The reflective metal film side contact layer 109 comprises three layers 111, 118 and 119 each of which comprises a material doped with different dopants. In these three layers, an interposed layer side contact layer 117 contacting to the interposed layer 108 comprises a material doped with Mg, an oxide layer side contact layer 119 contacting to the oxide layer 110 comprises a material doped with Zn, and an intermediate contact layer 118 provided between the interposed layer side contact layer 117 and the oxide layer side contact layer 119 comprises a material which is not positively doped.
The layers from the first electrode side contact layer 103 to the reflective metal film side contact layer 109 are referred as a double hetero structure part 120. In addition, the first cladding layer 105, the active layer 106, and the second cladding layer 107 may be totally referred as a light emitting layer 121.
In the semiconductor light emitting device 101 in FIG. 11, the light is not extracted from the main surface (opposite surface) where the supporting substrate 112 is provided, and the light is only extracted from another main surface formed on the light extract layer 104, by providing the reflective metal film 111.
The reflective metal film 111 disposed between the double hetero structure part 120 and the supporting substrate 112 has naturally a high reflectance with respect to the light emitted from the active layer 106, and is required to have an ohmic-contact to the double hetero structure part 120 mainly comprising AlGaInP based material. However, it is difficult to provide a direct ohmic-contact to the AlGaInP based material by using a metal such as Ag, Al, and Au that has a high reflectance at a wavelength of the light emitted from the active layer 106. Therefore, it is necessary to partially provide the ohmic contact portions 115 between the reflective metal film 111 and the double hetero structure part 120. Partial disposition means that the reflective metal film 111 is not totally covered but the ohmic contact portions 115 are disposed appropriately in the plane of the reflective metal film 111.
The ohmic contact portion 115 is disposed between the reflective metal film 111 and the double hetero structure part 120 to take the ohmic contact, in which the reflectance is low as compared with that of the reflective metal film 111. In addition, it is necessary to conduct heat-treatment after providing a material of the ohmic contact portion 115 to contact to the double hetero structure part 120, so as to take the ohmic contact. Alloying reaction occurs between the double hetero structure part 120 and the material of the ohmic contact portion 115 in the occasion of the beat treatment, so that a light absorption rate is increased in the double hetero structure part 120 contacting to the ohmic contact portion 115. For this reason, the light absorption is increased in the ohmic contact portion 115 compared with the non-ohmic contact portion 116, when the light emitted from the active layer 106 passes through the oxide layer 110. As a result, the light extract efficiency of the whole light emitting device is deteriorated.
Even though the reflective metal film 111 has a high reflectance with respect to the light emitted from the active layer 106, if much light cannot be extracted from a surface of the light extract layer 104 which is a main surface, the light extract efficiency will be deteriorated, so that the improvement in the light output will be small. Therefore, as a technique to extract the light effectively, it has been known to make the main surface rough (rough-surface treatment) as disclosed by Japanese Patent Laid-Open No-2004-356279. The rough-surface treatment is to form an irregularity (unevenness) on the surface.
When the light goes out from a substance, there is a constraint of a critical angle. If an angle of the light is perpendicular to a surface, the light can be extracted. However, when there is an inclination, the light cannot be extracted. The critical angle is determined based on a wavelength of the light and a refractive index of the substance. For example, among the light radiated from the light emitting layer 121, the light perpendicular to the light extract layer 104 goes out from the semiconductor light emitting device 101, however, the light having a predetermined angle with respect to the light extract layer 104 cannot go out from the semiconductor light emitting device 101, due to the angle with respect to the main surface. However, when the main surface is formed as a rough surface, the angle of the light having the predetermined angle with respect to the light extract layer 104 is changed with respect to the main surface, so that such a light goes out from the semiconductor light emitting device 101. Therefore, the light extract efficiency can be improved by making the main surface as a rough surface.
As a technique for improving the effect of the rough-surface treatment, there is a technique of forming an irregular pattern by photolithography which is a well-known technique. However, since minute pattern formation is required in this technique, expensive equipment is necessary, and a manufacturing cost of the semiconductor light emitting device is increased as consequence. In addition, the manufacturing cost is increased since the photolithography process is conducted. On the other hand, as a technique for reducing the manufacturing cost, there is a technique for conducting the rough-surface treatment without forming any pattern. However, there is a following disadvantage in this technique.
When the rough-surface treatment is conducted on a surface of the light extract layer which is the main surface, by etching without forming any pattern, the etching totally advances in accordance with the formation of the irregularity, so that a film thickness of the light extract layer is reduced totally. When the film thickness of the light extract layer is reduced totally, a current spreading is deteriorated, a forward voltage is increased, and a light output is decreased simultaneously. As a result, the light emitting efficiency is deteriorated.
The light output is decreased as follows. A calorific value in the semiconductor light emitting device is increased due to the increase in the forward voltage, so that the light emitting efficiency is deteriorated because of the influence of the heat thus generated. In brief, when the film thickness of the light extract layer is reduced, a series resistance is increased as well as the current spreading is deteriorated, so that the forward voltage is increased, thereby reducing the light output.